In the industrialized world of today, most homes and businesses are connected to telephone networks using twisted pair copper wires. Those copper wires were originally used solely to carry data traffic in the analogue voice band. However, with the advancement of technology, particularly Digital Subscriber Line (DSL) access technology, the transfer of data over the higher frequencies in the twisted pair copper wires was made possible. The greatest advantage of DSL is that it enables data to be exchanged over the twisted pair copper wires at much higher speeds than conventional modems and analogue lines. The speeds at which data is exchanged over DSL now exceed 200 Mbit/s downstream using the current access technology standard very-high-bit-rate digital subscriber line 2 (VDLS2). Despite high transmission rates of today, DSL access technology is still being developed towards even higher transmission rates. It has been agreed in ITU-T that the next generation of DSL access technology will be utilizing transmission technology that separates the different transmission directions in the time domain instead of separating the transmission directions in the frequency domain, as is the case for present DSL access technology. A working name used during current standardization efforts for the next generation of time-division duplex DSL-based access technology is “G.fast”.
DSL communication systems support high-speed data links between on the one side an operator modem or distribution point unit, possibly being part of Central Office equipment of a communication service provider, operator or network operator, and on the other side one or more residential network terminals serviced by the operator modem. Irrespective of used terminology, the two main pieces of equipment in a DSL communication system are a digital subscriber line access multiplexer (DSLAM) on the provider side of the communication service and numerous DSL modems on the customer side. In an ADSL or VDSL communication system, an available spectrum is subdivided into a plurality of tones, each of which carries either downstream information from the CO to a DSL modem or upstream information from the DSL modem to the CO. While the CO may provide service to a plurality of DSL modems, each serviced modem is coupled to the CO via a respective twisted pair of wires. Often, a large number of DSL connections are bundled together in one cable and then connected to a cabinet of other connecting terminals of an operator, network operator or other service provider.
The quality of the communication channel provided by a respective twisted pair or wires that couples a DSL modem to its servicing CO will have a substantial determination in the throughput that may be achieved between the CO and the DSL modem.
The quality of the communications channel between the CO and a serviced DSL modem depends upon a number of factors. One of these factors is distance, i.e. the distance from the CO to the DSL modem as signal attenuation increases with increased distance. Another factor is media quality, e.g. type of media, number of connections, etc. Still another factor is interference that may be coupled to the twisted pair or copper wires, often produced by a neighboring connection or by another DSL modem transmitting in the same frequency band. The interferences caused by the neighboring connections are usually referred to as crosstalk. Far-end crosstalk (FEXT) is caused by the far-end transmitters on neighboring lines, while near-end crosstalk (NEXT) is caused by the near-end transmitter on neighboring lines. Generally, NEXT is stronger than FEXT.
A DSL modem that powers up goes through initialization process including a synchronization procedure. The actual process varies from modem to modem but generally involves the steps of performing a self-test, checking the connection between the DSL modem and the computer or other customer equipment with which the DSL is connected, and finally synchronization with the DSLAM. Data can only be retrieved from the internet and thus reach a computer or other customer equipment when the DSL modem and the DSLAM are synchronized. The initialization process including synchronization is complex and involves tests that allow both sides of the connection to optimize the performance according to the characteristics of the line in use. Those tests relate to performance characteristics like modem capabilities, modes of operation, basic modulation parameters, etc.
The initialization procedure for setting up a connection between a DSL modem and a DSLAM according to the effective standard in ITU-T [G.994.1] will be described in more detail with reference to the following description. In DSL communication systems using time-division duplex structure, a problem related to the quality of communication is that a DSL modem during the start-up procedure is out of synchronization. Handshake signals in the beginning of the initialization procedure thus give rise to crosstalk that may cause disturbances to other existing communication channels. The interference caused could therefore deteriorate the quality of the communication channels. As mentioned, plural twisted pairs for DSL transmission are typically bundled together in the same cable binder and therefore, the crosstalk between neighboring lines may be a limiting factor for the communication service, its transmission capacity and quality.
To avoid that handshake signals during the start-up and initialization procedure will cause interference and disturb neighboring lines, the customer premises equipment must significantly reduce its output power. However, reduction of output power is not preferable, since the signal reach of the communication will then also be reduced, due to signal attenuation, i.e. a DSLAM might not be able to detect the initiation signal from a residential DSL modem if the distance there between is long enough.